To cross a stream, road or valley (exactly where the hold off caused by a flight of locks at either facet will be unacceptable) the valley might be spanned by a navigable aqueduct – a well-known example in Wales could be the Pontcysyllte Aqueduct (now a UNESCO Globe Heritage https://musica-infantil13211.suomiblog.com/the-fact-about-urso-fofo-that-no-one-is-suggesting-52805503